The temporal context of reinforcement carries substantial explanatory
weight in most theoretical perspectives. For example, the "scallop"
in operant theory is thought to be due to the temporal stimuli becoming
more like those at the moment of reinforcement in each sucessive portion of the interval.

Slide 2-49

This procedure varied the implict temporal stimuli while holding the explicit
clock stimuli constant. The clock stimuli were constant with respect
to the end of the interval or with respect to the beginning of the
interval. Each of several birds were consecutively exposed to the 4 procedures
of this experiment. Surprisingly the random end procedure maintained
increasing responding across the interval just like the CFT. This occured
even though the stimuli did not reliably specify the time to reinforcement.

Slide 2-50

The prior procedure had confounded the stimulus position with
probability of reinforcement. The probability of reinforcement had been
systematically correlated with key color
by randomly ending the clock but always restarting it at the beginning of the
sequence. In this experiment, the key color was not systematically correlated
with the probability of food because food was presented following a random
number of stimuli, but restared
at the next consecutive stimulus. The present experiment demonstrated 1) that
the prior results had been due to the probability of reinforcement asociated
with each color and 2) that stimuli that provide information concerning the
time since the prior food presentation do not control increasing rates of
responding. This experiment demonstrated this by exposing birds to a Random
Food schedule followed by a Clocked Fixed Time schedule and finally a Random
Food schedule.

Slide 2-51

This interpretation was proven by exposing birds to Random Stimuli Probabilistic
Food schedules (RSPF). Each of 3 birds was consecutively exposed to three
procedures with different randomly presented stimuli with a specific
probability of food associated with each stimulus, the color with the highest
probability is plotted furthest to the right, each bar to the left has a
lower probability of food. If each of a random series of stimuli is reliably
followed by reinforcement with a different probability, then the rate of
responding to each stimulus is a function of the probability of reinforcement.

Slide 2-52

In sum, responding was under the control of the relationship of the
stimuli with the reinforcer. Additionally, the time to reinforcement and
the probability of reinforcement similarly controlled responding.